Acier Miskina is the father of Olga Miskina and the genius scientist who created the legendary substance known as Pure Gold. He is a central figure in the special One Piece: Heart of Gold, where his past decisions and enduring love for his family drive the story's emotional core. Two hundred years before the events of the special, Acier was a relatively slender man with wild blonde hair tied back in a large bun. He typically wore a scientist's lab coat over a green shirt. After spending two centuries living among the dinosaurs inside the belly of the giant fish Bonbori, his appearance changed dramatically; he became much larger and heavier, his clothes were reduced to tatters, and he wore a homemade dinosaur costume to blend in with the creatures around him.

Personality-wise, Acier is defined by his profound loyalty to his family. His initial motivation for creating Pure Gold was entirely selfless: to halt the progression of a fatal illness, the South Blue Emperor Fever, which was afflicting his young daughter, Olga. This devotion persists even after two hundred years of isolation, as he breaks down in tears of joy upon learning that Olga is still alive. He is immediately willing to sacrifice himself to save her, even planning to detonate his own booby-trapped mine to prevent the treasure hunter Mad Treasure from obtaining the Pure Gold and harming his daughter. Despite his tragic past and grim circumstances, Acier also displays a quirky, eccentric side. He is a self-professed fan of "cool" things, like Franky's Franky Shogun robot. After his escape from Bonbori, he enthusiastically embraces the role of a slightly mad scientist, eagerly testing new inventions much to his daughter's chagrin.

In the narrative, Acier serves as both a guide and a tragic figure. When Monkey D. Luffy is separated from his crew and thrown into the same stomach of Bonbori where Acier has lived for two centuries, the two form an alliance. Acier uses his intimate knowledge of the ecosystem inside the monster to help Luffy traverse the dangerous environment, including taming a water strider lizard named Chavez, whose language he has learned to understand after decades of mimicking dinosaur behavior. His role evolves from a forlorn hermit to a proactive father determined to correct the mistakes of the past. The core of his development revolves around his relationship with Olga. For two hundred years, Olga harbored a deep resentment toward her father, blaming his greed for the Pure Gold for the death of her mother, Livia, and for their subsequent imprisonment inside Bonbori. However, during the climax of the special, Olga discovers her father's old journals and learns the truth: he never sought wealth, but only a cure for her illness. This revelation allows father and daughter to finally reconcile, and Acier expresses his simple wish to live a happy, peaceful life with her.

As a scientist, Acier possesses remarkable abilities. His greatest achievement is the creation of Pure Gold, a substance with properties that grant a form of near-immortality to its wearer by halting the aging process. He is also a skilled engineer, having designed a complex, trap-filled mine on the island of Alchemi to safeguard his creation. Despite his intellectual brilliance, Acier is physically very fragile, acknowledging that even a single punch from Luffy could be fatal to him. His long years of survival have also granted him the unique ability to communicate with the dinosaurs and other reptiles living within Bonbori's stomach, a skill that proves essential for navigation and transportation. After the events of the special, he invents a weight-loss serum that restores him to his original, slimmer physique, which he later markets as a commercial product.